1. Field of the Invention
The present invention relates to a power control method applicable to mobile communication systems, and more particularly, to a transmission power control apparatus and method using reverse channel quality indicator and acknowledgment indicator.
2. Discussion of the Related Art
In radio communications, channel environment varies according to the drift of mobile terminal's location. Hence, it is preferable that the modulation and coding scheme are modified to fit the channel quality for each situation.
With regard to setting a modulation scheme, when the channel quality is good (i.e., less interference), the communication system is able to use modulation enabling high-speed data transfer, such as QAM (quadrature amplitude modulation) and M-ary PSK (phase shift keying). However, in case that channel quality is poor, it is able to use such modulation as BPSK (binary phase shift key) resistant against interference.
With regard to setting a coding scheme, when the channel quality is good, less redundancy (thus, a high coding rate) is possible, so that data can be transmitted with higher data rate. However, when the channel environment is poor, the channel coding is performed with more redundancy (lower coding rate), so that data can be transmitted with a lower data rate.
In order to vary the modulation and coding scheme appropriately according to the variation of the channel quality, information about the current channel quality is needed. A forward channel quality is measured by a mobile terminal and is transmitted to a base station via a reverse channel quality indicator channel (R-CQICH). It should be noted that the term reverse channel is denoted as communication originating from a mobile terminal and transmitted to a network, such as a base station.
H-ARQ (hybrid automatic repeat request) is a method for improving reliability and throughput in a manner of combining ARQ (automatic repeat request) and FEC (forward error correction). ARQ is a method for improving transmission reliability in a manner of requesting retransmission of the same information until receiving errorless information if error exists in the transmitted information. And, FEC is a method for improving reliability in a manner of correcting errors having occurred during transmission.
During good channel quality, the frequency of errors in the received information is low. Hence, a retransmission is requested using ARQ, whereby reliability of the received information can be maintained. However, during poor channel quality, the frequency of errors in the received information is high. If ARQ is used without FEC, it may cause the increase of the number of retransmissions. Hence, the throughput of the system will be decreased since ARQ does not have any error-correction function.
Since such a problem can be solved by FEC, the H-ARQ system using both ARQ and FEC has been proposed. As one sort of H-ARQ, there is the IR (incremental redundancy) system. In the IR system, a transmitting side initially transmits data encoded with high coding rate which have small number of redundant bits. If the receiving side receives data with errors, it requests retransmission. In response to the request, the transmitting side transmits additional redundant bits, which are caused by low rate encoding.
A receiving side combines to decode the already received data and the redundant bits. In doing so, the retransmitted bits are to compensate the previously sent packet.
In the HARQ system of a wireless communication system, a mobile terminal decodes a received packet to check a presence or non-presence of errors and should feed back an ACK (acknowledgment) or NAK (negative acknowledgment) signal to a base station according to a result of the check. A base station having received the NAK signal retransmits the packet. By combining to decode the retransmitted packet and the initially transmitted packet, the mobile terminal has a diversity or coding gain. The ACK/NAK signal transmitted from the mobile terminal to the base station is transmitted to the base station via a reverse acknowledgment channel (R-ACKCH).
In a typical wireless communication system, nominal attribute gain for R-ACKCH is set to −3 dB. During the course of implementation, it has been determined that this gain was set too low for a proper ACK operation. In other words, current nominal attribute gain for R-ACKCH makes false alarm probability (probability that the base station receiver detects ACK even when the transmitter does not transmit anything on R-ACKCH) too high resulting in a large number of RLP retransmissions.
To identify the problem, simulations were performed with current nominal adjustment gain value for R-ACKCH under AWGN channel. The simulations were performed with 9600 bps R-FCH on top of R-ACKCH. The pilot level was power controlled so that 1% FER could be achieved for R-FCH.
In FIG. 1, the line 2 represents the CDF of demodulator output when ACK signal is transmitted. The line 4 is the complementary CDF of demodulator output when the transmitter does not transmit anything. The line 6 is the complementary CDF of demodulator output when NAK signal is transmitted.
For ease of explanation, the following probabilities can be defined.
PA-N: Probability that the ACK signal is falsely detected as NAK.
PN-A: Probability that the NAK signal is falsely detected as ACK.
PNo-A: Probability that the receiver detects ACK signal even when the mobile station doesn't transmit anything on R-ACKCH.
In this example, it is assumed that one threshold is given to the output of base station demodulator so that the base station detects ACK or NAK. It should be noted that ‘no signal’ does not need to be differentiated from ‘NAK’ since the base station behavior might be the same for these two cases. The criteria used for determining the threshold level is to maintain the PA-N and PN-A below certain level. The choice of this level should be implementation dependent. However, PA-N of 0.01 seems to be reasonable choice. From FIG. 1, PN-A is 0.001 for this threshold, which seems quite reasonable. However, it turns out that PNo-A is 0.3, which is a bit high for proper operation. The high PNo-A may lead to some erroneous event.
The erroneous event due to this false alarm on R-ACKCH can be explained as follows. When a mobile terminal completely misses the forward packet data control channel given to it, the mobile terminal will not transmit any signal on the R-ACKCH. For about 30% of these situations, the base station falsely decides that ACK signal was transmitted from the mobile terminal and is going to proceed with new packet for that ARQ channel, resulting in RLP layer retransmission for that packet. Therefore, it is suggested that the channel gain (i.e., transmission power) for R-ACKCH needs to be modified to resolve this problem.
Transmission power of R-CQICH is determined using a R-CQICH power adjustment gain and a R-ACKCQICH gain to a pilot power (RLGAIN_ACKCQICH_PILOT). The R-CQICH power adjustment gain is individually transmitted to each mobile terminal from a base station. And, the R-ACKCQICH gain to a pilot power is commonly transmitted to the all mobile terminals from the base station.
Similarly, transmission power of R-ACKCH is determined using a R-ACKCH power adjustment gain and a R-ACKCQICH gain to a pilot power (RLGAIN_ACKCQICH_PILOT). The R-ACKCH power adjustment gain is individually transmitted to each mobile terminal from a base station. And, the R-ACKCQICH gain to a pilot power is commonly transmitted to all the mobile terminals from the base station.
As mentioned in the foregoing description, in determining each of the R-CQICH and R-ACKCH transmission powers, the R-ACKCQICH gain to a pilot power (RLGAIN_ACKCQICH_PILOT) is commonly used. However, because the common factor (RLGAIN_ACKCQICH_PILOT) is used in determining both the R-CQICH and the R-ACKCH transmission powers, the following problems are inevitable.
For all mobile terminals in a cell, it may occur that the transmission power of R-ACKCH needs to be increased but the transmission power of R-CQICH need to be maintained. In such a case, the R-ACKCH power adjustment gain should be transmitted to all mobile terminals in the cell individually.
This is because the transmission power of R-CQICH is increased as well as that of R-ACKCH, if the R-ACKCQICH gain to a pilot power (RLGAIN_ACKCQICH_PILOT) is transmitted to all mobile terminals in a cell using an overhead message.
For instance, in detecting ACK/NAK, the base station performs a threshold detection using the receiving power of R-ACKCH. Hence, in case that the transmission power of R-ACKCH is too small, the base station may incorrectly detect No-signal as NAK. If the R-ACKCQICH gain to a pilot power (RLGAIN_ACKCQICH_PILOT) is transmitted to all mobile terminals to solve the problem, the transmission power of R-CQICH is unnecessarily increased to be inefficient. Meanwhile, if the base station transmits the R-ACKCH power adjustment gain to each of the mobile terminals, a message load transmitted to the mobile terminal increases and the corresponding transmission process gets very complicated.